Recuperated atmospheric SOFC/gas turbine hybrid cycle

ABSTRACT

A method of operating an atmospheric-pressure solid oxide fuel cell generator ( 6 ) in combination with a gas turbine comprising a compressor ( 1 ) and expander ( 2 ) where an inlet oxidant ( 20 ) is passed through the compressor ( 1 ) and exits as a first stream ( 60 ) and a second stream ( 62 ) the first stream passing through a flow control valve ( 56 ) to control flow and then through a heat exchanger ( 54 ) followed by mixing with the second stream ( 62 ) where the mixed streams are passed through a combustor ( 8 ) and expander ( 2 ) and the first heat exchanger for temperature control before entry into the solid oxide fuel cell generator ( 6 ), which generator ( 6 ) is also supplied with fuel ( 40 ).

GOVERNMENT CONTRACT

The Government of the United States of America has rights in thisinvention pursuant to Contract No. DE-FC 26-97FT34139, awarded by theU.S. Department of Energy.

FIELD OF THE INVENTION

The present invention relates to an integrated gas turbine solid oxidefuel cell system, where inlet air for a solid oxide fuel cell generatoris slightly above atmospheric and where inlet air for the generator istemperature controlled to increase flexibility in selecting a gasturbine for integration with the solid oxide fuel cell (“SOFC”)generator.

BACKGROUND OF THE INVENTION

Typically, a solid oxide fuel cell generator comprised a plurality oftubular solid fuel cells that react a gaseous fuel, such as reformednatural gas, with air to produce electrical power and a hot exhaust gas.Previously, it has been proposed to integrate such a solid oxide fuelcell generator with a gas turbine, where electrical power is produced byboth the solid oxide fuel cell generator and the turbine. Such a systemwas used with a topping combustor supplied with a second stream of fuelto provide a still further heated hot gas that was then expanded in aturbine, as taught in U.S. Pat. No. 5,413,879 (Domeracki et al.).

There, the system is a pressurized-SOFC-generator/gas turbine (PSOFC/GT)hybrid system. It gets high efficiency (relative to the efficiency of aconventional SOFC power system) because it recovers SOFC exhaust heatand converts a fraction of that heat to electric power, and because itoperates the SOFC generator at elevated pressure, which boosts cellvoltage, which means the SOFC generator runs at a higher efficiency.However, such a system is both complex and expensive.

A wide variety of integrated SOFC/gas turbine systems have beenproposed, in, for example, Proceedings of the Power-Gen International'96, “Solid Oxide Fuel Cell/Gas Turbine Power Plant Cycles andPerformance Estimates”, Wayne L. Lundberg, Dec. 4-6, 1996 Orlando Fla.;and U.S. Pat. No. 5,573,867 (Zafred et al.). A variety of integrateddesigns have also been used in molten carbonate fuel cell technology,for example, U.S. Pat. Nos. 3,972,731 and 4,622,275 (Bloomfield et al.and Noguchi et al. respectively).

More recently Stephen E. Veyo and Wayne L. Lundberg et al. in theProceedings of ASME Turbo Expo 2003, “Tubular SOFC Hybrid Power SystemStatus”, Jun. 16-19, 2003, Atlanta Ga., described current designatmospheric-pressure SOFC/gas turbine (“ASOFC/GT”) hybrid cycle systems,as well as a turbocharged SOFC hybrid cycle, among others. There, in theASOFC/GT design (shown in FIG. 1) cycle air is taken in at a gas turbinecompressor, and preheated with SOFC exhaust heat recovered at a singlerecuperator. This reduces the gas turbine combustor fuel requirement toachieve a prescribed turbine inlet temperature (TIT), and raises thesystem cycle electric efficiency. The oxidant for the SOFC module is theturbine exhaust, which will typically be at a pressure that isapproximately 1-3 psi above the atmospheric pressure. Thus, an advantageof the system is that the module will not require the complication andexpense of design for pressurization, and a module with features from aconventional atmospheric-pressure SOFC power system could be employed.

Systems based on the ASOFC/GT cycle will not be limited to a particularelectric power capacity, and it is expected that capacities ranging fromcirca 100 kwe's to multi-MWe's will be feasible. Further, electricefficiencies (net AC/LHV) of approximately 52% are expected fromASOFC/GT systems. The system could also incorporate a heat exportfeature, giving it combined heat and power capabilities. For an ASOFC/GTsystem configured as shown in the article (and FIG. 1), the gastemperature at the turbine expander exhaust must be the oxidanttemperature that is required at the SOFC module inlet, and for this tooccur, the GT pressure ratio and TIT at GT rating will therefore berestricted to combinations that will result in the required module inlettemperature, or the GT must be operated off-design to achieve therequired module inlet temperature. This could limit the number ofcommercially-available gas turbines that are suited for deployment in aASOFC/GT system of specified capacity, and if GT off-design operationwere required to use a particular GT, reduced system power andefficiency performance could result. Thus, a design issue with powersystems that are based on the basic ASOFC/GT cycle is that it isdifficult to go out and buy a gas turbine that will provide exactly theair temperature and air flow rate combination that the SOFC generatorneeds at its inlet to keep its cells running at the right temperature.

What is needed is a modification to the basic ASOFC/GT cycle that wouldenable the application of gas turbines that did not operate with thepreferred expander exhaust temperature under rating conditions, and itcould preclude the need to operate the GT off-design for expanderexhaust temperature control purposes. It is one of the main objects ofthis invention to provide a modification to the system that wouldfacilitate easy control of the oxidant temperature between the gasturbine and an associated SOFC module. There is a need to allow the gasturbine in ASOFC/GT made to be less dependent on SOFC module operationalrequirements.

SUMMARY OF THE INVENTION

The above needs are met and object achieved by providing a method ofoperating an atmospheric-pressure solid oxide fuel cell generator incombination with a gas turbine comprising a compressor and expandersection; where an inlet oxidant is passed through the compressor andexits as a first stream and a second stream, the first stream passingthrough a flow control valve to control flow and then through a heatexchanger, followed by mixing with the second stream; where the mixedstreams are passed through a second heat exchanger, and then a combustorand the expander section of the gas turbine before entry into the solidoxide fuel cell generator, which generator is also supplied with fuel.It is through control of air/oxidant flow through the flow control valveand use of the first heat exchanger that control of the SOFC generatorair inlet temperature is achieved.

This invention also resides with an atmospheric-pressure solid oxidefuel cell generator in combination with a gas turbine having acompressor and an expander, where inlet oxidant passes into the gasturbine comprising a compressor and expander section, and exits as afirst oxidant stream and a second oxidant stream, where the first airstream passes through a flow control and into a first heat exchanger tobe heated and then into the second stream to form a flow controlled,combined, compressed heated oxidant stream which passes to a second heatexchanger to be further heated, to form a flow controlled, furtherheated oxidant stream which is passed through a combustor and which ispassed into the expander of the gas turbine to form an expanded, cooledoxidant stream which is passed through another heat exchanger,preferably the first heat exchanger, to be further cooled, to provide aflow-controlled, heat-adjusted, feed oxidant stream, having thetemperature and pressure required to be a fuel cell generator oxidantfeed, which is then passed to a solid oxide fuel cell generatorcontaining a plurality of tubular solid oxide fuel cells havingelectrodes on opposite sides of a solid oxide electrolyte which fuelcells operate on oxidant and fuel to generate electricity, effectingadjustment and control of the feed air stream in terms of temperature atthe oxidant inlet.

This system will provide the system designer with more flexibility,control and options by increasing the variety/number of gas turbinesthat are candidates for deployment in a ASOFC/GT system. The flowcontrol valve coupled with the first heat exchanger (recuperator)controls temperature at point of entry in the SOFC. The term“atmospheric-pressure” as used herein means from 1.0 to about 1.2atmospheres (14.7 psia-17.7 psia). The temperatures at the SOFCgenerator air inlet will be in the range of about 500° C. to about 700°C., depending upon the SOFC generator operating point. The air mass flowrate at the SOFC generator air inlet will be 1 kg/sec and higher,depending upon the power system capacity. The bigger the capacity, thehigher the flow requirement.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of this invention will be more apparentfrom the following description in view of the drawings, where:

FIG. 1 is a schematic diagram of a prior art, basic ASOFC/GT powersystem cycle;

FIG. 2 is a schematic diagram of an oxidant temperature controlledASOFC/GT power system cycle according to this invention; and

FIG. 3 is a simplified three dimensional view of a tubular solid oxidefuel cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, there is shown in FIG. 1 an integrated gasturbine solid oxide fuel cell system according to the prior art. Themajor components of the system are a gas turbine generator with acompressor 1, a turbine expander 2, a rotor 12 by which the turbinedrives the compressor as well as an electrical generator 9, an optionalfuel compressor/pump 3, a fuel desulfurizer 5, a solid oxide fuel cell(SOFC) module generator 6, an optional combustor 8 and recuperator 4,with fuel feeds shown as 40 and 70 and feed air/oxidant shown as 20.Various fuel line valves 50 are shown, as well as power conditioningsystem 52, commercially available, that converts SOFC DC power to ACpower.

The SOFC generator module 6 operates at near atmospheric pressure, as ina conventional SOFC power system. The turbine is indirectly heated byheat recovered from the SOFC module exhaust 24, but the combustor 8could be also fired for system peak-power operation. The SOFC modulereceives its module oxidant input 22 from the turbine expander 2exhaust. Thus, when the turbine combustor is not fired, the oxidant willbe normal air, and vitiated air when it is fired. A system based on thiscycle will derive electric power from both the SOFC electrochemicalprocess and the conversion of SOFC exhaust heat and gas turbine fuelenergy to power by the gas turbine. The principal advantage of theatmospheric-pressure hybrid is its ability to achieve moderately highelectric efficiencies while still employing the simpler and lessexpensive atmospheric-pressure SOFC module package design.

Here, cycle feed air 20 is taken in at the compressor 1, and preheatedwith SOFC exhaust heat recovered at the recuperator 4. This reduces thegas turbine combustor fuel requirement to achieve a prescribed turbineinlet temperature, and raises the system cycle electric efficiency. Themodule oxidant 22 for the SOFC module 6 is the turbine exhaust, whichwill typically be at a pressure that is approximately 1 to 3 psi abovethe atmospheric pressure. Thus, an advantage of the system is that themodule will not require the complication and expense of design forpressurization, and a module with features from a conventionalatmospheric-pressure SOFC power system could be employed.

For an ASOFC/GT system configured as indicated in FIG. 1, the gastemperature at the turbine expander section 2 exhaust must be theoxidant temperature that is required at the SOFC module inlet 41, andfor this to occur, the GT pressure ratio and turbine inlet temperatureat GT rating will therefore be restricted to combinations that willresult in the required module inlet temperature, or the GT must beoperated off-design to achieve the required module inlet temperature.This could limit the number of commercially-available gas turbines thatare suited for deployment in a ASOFC/GT system of specified capacity.

Referring now to FIG. 2, which shows the new ASOFC/GT system of thisinvention, the addition of another recuperator 54 and the flow controlvalve 56 feature provides for the modulation and control of the oxidanttemperature at the SOFC module inlet 41. It is possible that flowcontrol could also be achieved by a flow control valve (not shown) inline 62. Using a module inlet temperature setpoint, the recuperator54-flow control valve 56 would be designed to automatically adjust itsposition as needed to divert relatively cool compressor discharge air 60to the recuperator 54 air inlet, and to affect the required adjustmentin the recuperator 54 exit oxidant stream 22 exhaust temperature. Thiswill cause the gas turbine operational characteristics in ASOFC/GT modeto be less dependent on SOFC module operational requirements. As aresult, the method is effective to provide the system designer with moreflexibility and options by increasing the number of gas turbines thatare candidates for deployment in a ASOFC/GT system, and it will enablethe hybrid system performance to be better optimized for highperformance by weakening or eliminating the dependency of the SOFCmodule inlet air temperature on GT operating conditions.

Thus, in FIG. 2, a gas turbine having compressor 1 and an expander 2,has inlet oxidant (air) 20 passing into the compressor 1 and exiting asa first oxidant (air) discharge stream 60 and a second air stream 62.The first air stream 60 passes through a flow control valve 56 and intoa first heat exchanger/recuperator 54, to countercurrent cool expandedexhaust stream 58, to provide adjusted air stream 22. Stream 60 thenmerges into the second air stream 62 at point/junction 64 to form acombined, flow controlled heated oxidant stream 66. This stream 66 ispreheated due to heat it picks up at first recuperator 54 and thenpasses to a second heat exchanger/recuperator 4, where it iscountercurrent heated by exhaust 24 from the SOFC generator to form aflow controlled further heated oxidant (air) stream 68 which is passedthrough a combustor 8 supplied with fuel from stream 70. Then the flowcontrolled further heated oxidant is passed into the expander 2 of thegas turbine to form an expanded, cooled, flow controlled oxidant “air”stream 58. When the gas turbine combustor is fired, the gas 58 exitingthe expander is no longer air but it's had its oxygen concentrationreduced somewhat, and it contains some CO₂ and more water vapor.

Stream 58 is now relatively cool, and is passed through first heatexchanger/recuperator 54 to provide a flow-controlled, heat-adjustedfeed oxidant stream 22 which is passed to the oxidant inlet 41 of asolid oxide fuel cell generator 6 containing a plurality of solid oxidefuel cells 72 (one shown for simplicity) arranged in fuel cell bundles,each fuel cell having electrodes on opposite sides of a solid oxideelectrolyte, which fuel cells operate on oxidant from SOFC inlet 41 andfuel 40 to generate electricity. A simple example of a fuel cell, forexample a tubular SOFC 72, is shown in FIG. 3 with electrolyte 74 airelectrode 76, fuel electrode 77, interconnection 78, oxidant flow 80 andfuel flow shown as the outside arrow, as is well known in the art.

A plurality of these fuel cells can be used in a SOFC generator havingmeans for receiving said slightly pressurized (1-3 psi gauge) air fromexpander exhaust, means for receiving a flow of fuel, and means forreacting at least a first portion of said received flow of fuel withsaid air so as to produce electrical power and a hot exhaust gascontaining oxygen. The SOFC/gas turbine system contains a) acompressor/expander for producing compressed expanded air; b) a solidoxide fuel cell generator having (i) an oxidant/air inlet manifold inflow communication with the expander, (ii) a fuel inlet manifold havingmeans for receiving a first flow of fuel, (iii) a plurality of solidoxide fuel cells in flow communication with the air inlet manifold, (iv)a reaction chamber in flow communication with the fuel inlet manifoldand in which the solid oxide fuel cells are disposed, such that thesolid oxide fuel cells cause fuel to react with air so as to produceelectrical power and hot exhaust gas, (v) at least two oxidant flowstreams (vi) at least two heat exchangers and (vii) at least one flowcontrol valve; where the valve controls first oxidant/air flow prior toheating in a first heat exchanger and mixing with a second oxidant/airflow which mixed oxidant is passed to a second heat exchanger.

The additional recuperator 54 and flow control valve 56 hardware shownin FIG. 2 will add pressure drop to the system which will tend to reducethe system net AC power output and efficiency. However these effects canbe made minimal by design by minimizing the flow resistance offered bythe recuperator and the flow control equipment. The additional hardwarewill also increase the system cost, which will be at least partiallycompensated for by the elimination of restrictions on gas turbineselection and operation which could improve system power and efficiencyperformance.

Also, as shown in FIG. 2, high temperature SOFC exhaust 24, at about800° C.-850° C., which is the product of depleted-fuel combustion andheat exchange with air that has entered the SOFC generator at point 41passes through recuperator 4 countercurrent to flow controlled oxidant66. Upon exiting recuperator 4, this exhaust may then be passed to aheat export heat exchanger 82 to heat water 84 to provide hot water 86as well as cooled exhaust 88. Water pump 90 is also shown. Heat exportheat exchanger 82 could heat water, but it could also produce steam forsite use, or for use by a steam driven absorption chiller to makecooling effect, or an absorption chiller could be installed at location82 that would operate directly on heat recovered from the hot SOFCexhaust. These heat applications would be good for small multi-hundredkWe or multi-MWe cogeneration systems for use at large office buildings,shopping centers, etc. Or, steam produced at location 82 could be usedto drive a steam turbine bottoming cycle, which would make more electricpower, and result in an even higher system electric efficiency. Also asshown, in first recuperator 54, first oxidant compressor dischargestream 60 is passed through flow control valve 56 and thencountercurrent to expander discharge oxidant 58.

While it is true there is a need for a certain rate of air flow to theSOFC generator, that is achieved by controlling gas turbine rotationalspeed. Valve 56 adjusts the flow split between parallel lines 60 and 62,and is not used to achieve overall flow control at points like 22 and41. The gas turbine operating point (speed of rotation) will set theflow (just like a pump), and the SOFC flow resistance will set thepressure at the SOFC-but it will be close to atmospheric pressure sinceit is known that the pressure drops through the SOFC generator and therecuperator 4 are small. Again, the invention hardware (line 60, valve56, recuperator 54) are intended only to control temperature in line 22and at point 41. Flow control in this invention only pertains to theflow split between lines 60 and 62. Overall control of the flow of airto the SOFC generator, as noted above, is achieved by the gas turbinespeed control.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

1. A method of operating an atmospheric-pressure solid oxide fuel cellgenerator in combination with a gas turbine comprising a compressor andexpander section; where an inlet oxidant is passed through thecompressor and exits as a first stream and a second stream, the firststream passing through a flow control valve to control flow and thenthrough a first heat exchanger, followed by mixing with the secondstream; where the mixed streams are passed through a second heatexchanger, and then a combustor and the expander section of the gasturbine before entry into the solid oxide fuel cell generator, whichgenerator is also supplied with fuel.
 2. The method of claim 1, whereinthe mixed streams after exit from the expander section are passedthrough the first heat exchanger, where control of oxidant flow throughthe flow control valve and use of a first heat exchanger control thetemperature of the mixed stream at entry into the solid oxide fuel cellgenerator.
 3. The method of claim 2, wherein the temperature of themixed streams at entry into the solid oxide fuel cell generator will bebetween about 500° C. to about 700° C.
 4. The method of claim 1, whereinthe operating pressure of the solid oxide fuel cell generator is fromabout 1.0 atmospheres to about 1.2 atmospheres, and a flow control valveis also present in the second stream.
 5. The method of claim 1, whereinthe method is effective to provide flexibility by increasing the numberof gas turbines that are candidates for use in the method.
 6. A methodof operating an atmospheric-pressure solid oxide fuel cell generator incombination with a gas turbine comprising a compressor and expandercomprising: (a) passing inlet oxidant into a gas turbine comprising acompressor and expander section; and then (b) exiting the oxidant fromthe gas turbine compressor as first and second oxidant streams; and then(c) passing the first oxidant stream through a flow control valve andthen into a first heat exchanger to heat the first oxidant stream; andthen (d) combining the first and second oxidant streams to provide acombined, compressed heated oxidant stream which is passed to a secondheat exchanger to be heated to provide a flow controlled, further heatedoxidant stream; and then (e) passing the flow controlled further heatedoxidant stream through a combustor and the expander section of the gasturbine, and then (f) passing the flow controlled, further heatedoxidant stream through a heat exchanger to cool the stream to provide aflow controlled-heat adjusted, solid oxide fuel cell feed oxidantstream; and then (g) passing the flow controlled-heat adjusted, feedoxidant stream into an atmospheric-pressure solid oxide fuel cellgenerator which generator is also supplied with fuel.
 7. The method ofclaim 6, wherein the pressure of the solid oxide fuel cell is from about1.0 atmospheres to about 1.2 atmospheres.
 8. The method of claim 6,wherein the flow controlled, further heated oxidant stream is passed, instep (f) through the first heat exchanger counter current to the firstoxidant stream after that stream passes through the flow control in step(c).
 9. The method of claim 6, wherein, fuel is also fed to thecombustor in step (e).
 10. The method of claim 6, wherein, in step (f)the oxidant stream feed is at a pressure of from about 1.1 atmospheresto about 1.2 atmospheres and at a temperature between about 500° C. andabout 700° C.
 11. The method of claim 6, wherein the solid oxide fuelcell generator contains a plurality of tubular solid oxide fuel cellshaving electrodes on opposite sides of a solid oxide electrolyte, whichfuel cells operate on oxidant and fuel to generate electricity.
 12. Themethod of claim 6, wherein, a feed fuel is passed through a desulfurizerand then into the solid oxide fuel cell generator.
 13. The method ofclaim 11, wherein, oxidant air and fuel, after contact with the fuelcells, form high temperature exhaust gas which is passed into the secondheat exchanger in step (d) to counter current heat the combined oxidantstream.
 14. The method of claim 13, wherein, the exhaust passing fromthe second heat exchanger is passed to a heat export heat exchanger toheat water or to provide steam.
 15. The method of claim 6, wherein theoxidant is air and the fuel comprises natural gas.
 16. The method ofclaim 6, wherein the temperature of the feed oxidant stream of step (g)is adjusted and controlled to make each of a variety of candidate gasturbines be compatible with a specific SOFC generator.
 17. The method ofclaim 6, wherein control of oxidant flow through the flow control valveand use of at least one heat exchanger control the temperature of themixed streams at entry into the solid oxide fuel cell generator.
 18. Themethod of claim 6, wherein the method is effective to provideflexibility by increasing the number of gas turbines that are candidatesfor use in the method.